This invention relates to an adaptive threshold circuit and, more particularly, to an adaptive threshold circuit for use with magnetic or variable reluctance sensors that produce an alternating voltage in response to rotation of a wheel.
Magnetic sensors are used in automotive applications to provide timing signals for the determination of, for example, engine crankshaft position and speed, and in anti-lock brake systems for determining wheel speed. This type of sensor is adjacent to a driven wheel that has circumferentially spaced slots. The wheel is associated with a magnetic pick-up that includes a pick-up coil and a permanent magnet. As the wheel rotates relative to the pick-up, an alternating voltage is generated in the coil.
A disadvantage of a magnetic type of sensor is that the peak voltages generated are proportional to the speed of rotation of the wheel. Thus, the peak voltage generated can vary, for example, from 250 millivolts (mvolts) at low speeds, to over 160 volts at higher speeds. Some sensors are rated up to 200 volts. In order to correctly decode the generated signal, the receiving circuit must adapt the threshold voltage it uses to recognize a positive or high voltage level.
As mentioned, the voltages generated in the pick-up coil are generated when a slot on the wheel moves past the sensor. Any dirt or scratches on the surface of the wheel will generate output noise that is also proportional to the Speed of the wheel. At high speeds, this noise can be several volts in amplitude.
As one possible solution to the noise problem, it has been proposed to utilize an analog circuit that has adaptive threshold control. One example of such a circuit is shown in the U.S. Pat. No. to Christenson et al., 5,144,233, granted on Sep. 1, 1992.
Analog adaptive controls have some drawbacks. Thus, first of all, they require an external capacitor to store the voltage for the next threshold. The charge must be stored for several milliseconds, which requires a sizable capacitor to maintain accuracy. Second, unwanted noise spikes in the system can modify the charge stored on the capacitor. This, in turn, causes inaccuracies with the next input switchpoint. Third, since the circuit is mostly an analog circuit, processing variations will cause parametric changes in operation of the circuit.